Throughout my career with University of Missouri Extension, there have been occasions where I have seen herbicide injury on crop plants.  Some of these were slight, but many were serious, with significant crop loss.

There are several types of herbicide injury.  A common one is off-target drift.  Many herbicides are quite volatile, and can drift for many miles, injuring crops and gardens.  Drift from 2,4-D applications is a good example of this kind of problem. 

A second type of herbicide injury is carryover.  In this case, the herbicide which was applied to a crop the previous season does not break down completely, and causes damage to the crop you have planted this year.

You can also see herbicide injury if your spray equipment is not cleaned out completely.  Some chemicals have a notorious reputation for being hard to clean from equipment, and yes, I have seen that kind of damage on several occasions as well.

Occasionally, you can see damage with labeled applications of herbicides… chemicals being used on the crop that they were designed and labeled for…. but for some reason things go wrong and they still cause injury.  That is rare, but it still happens.  Just follow the label to the letter and that will usually keep you from problems.

But the worst cases that I have seen have been herbicides that were not labeled for the crop that they were being used on.  If a non-labeled herbicide is applied to a crop, that is in violation of the law.  It’s also playing with fire and you risk damaging your crop.  I’ve not seen that so far in Northwest Missouri, but I occasionally would see that in my previous position.  And it often caused significant damage.

Another situation sometimes arises where a herbicide that is not safe for a crop is applied in a setting which intensifies the damage.  Two examples come to mind that I have seen in Northwest Missouri greenhouses.

In these situations, growers were trying to eliminate weeds on the greenhouse floor.  The grower would spray the floor, to keep the weeds at bay.  That worked, but unfortunately the herbicides vaporized from the floor and those vapors killed or injured sensitive plants.  This was very costly, needless to say.

The bad news is that the vapors didn’t go away.  In a case last year, it was a warmer spring, and by opening the side curtains, the greenhouse could air out and most of the vapors were removed.  This year, with a cold spring and closed-up greenhouse, the problem that I saw was more serious.  I talked to our state Floriculture Specialist, Dr. David Trinklein, and he said that the only remedy would be to use activated charcoal to absorb the vapors, or to dig up the top 6 inches of soil where the herbicide was, and remove the soil. 

The greenhouse decided on the latter option, which worked very well.  The plants look great now, but there was obviously some loss and a lot of time spent in the process.

So be careful with herbicides in greenhouses.  If you have any doubt, please give me a call before you apply them.